Multiple arc plasma device with continuous gas jet

ABSTRACT

A multiple cathode DC arc plasma generator arrangement is used in connection with a single anode for thermal arc plasma processing of materials. A nozzle is provided to introduce a gas in approximately the center of the multiple cathodes, towards the anode. The nozzle injects the gas into the center of the plasma column generated between the cathodes and anode to stabilize such arc and affect the self-induced electrode jets. This provides control of the heat transfer to the anode and permits feeding of particulate matter into the core of the plasma column to enhance inflight processing (melting and/or chemical reaction) of the matter. A set of gas nozzles positioned radially about the anode may be employed for feeding of particulate matter at the anode surface.

This invention was made with Government support under Grant Number CPE82-00628 awarded by the National Science Foundation. The Government hascertain right in this invention.

CROSS REFERENCE TO RELATED APPLICATION

This application is a division of application Ser. No. 08/817,759, filedJan. 6, 1986, and now U.S. Pat. No. 4,725,447, which application in turnwas a continuation of application Ser. No. 655,340, filed Sept. 27,1984, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to continuous plasma processing ofmaterials, and more particularly to an apparatus and method for plasmaprocessing which utilizes a gas injected in the center of a coalescedplasma column for obtaining desired heat transfer to the anode and/orparticulate material processing.

2. Description of the Prior Art

Thermal plasmas, formed by arcs between a cathode and an anode, generatehigh temperatures and have an extremely active nature, and thus haveattracted the interest of metallurgists and chemical synthesists foryears. High intensity arcs are used for welding, cutting, plasmaspraying, lighting, interrupting high power circuits, melting andalloying, and producing ultra fine refractories in the form ofparticulate material. It is known that arc induced magnetohydrodynamiceffects dominate a high intensity arc. Interaction of the arc currentwith the self magnetic fields gives rise to a pumping action, inducingjets or gas flows called a cathode jet and an anode jet. The action ofsuch induced gas flows depends on the gap between the cathode and anodeand on the current strength, and can substantially affect the total heattransfer to the anode.

The present invention uses a multiple arc system, in the preferred form.In conjunction with the multiple arc system known electrical circuity isused as explained in an article by J. E. Harry and R. Knight entitled"Simultaneous Operation of Electric Arcs From The Same Supply", IEEETrans Plasma Science, PS. 9(4) pp. 248-254 (1981), and also in anarticle "Power Supply Design For Multiple Discharge Arc Processes",published in the 6th International Symposium on Plasma Chemistry,Symposium Proceedings, Volume 1, pp. 150-155 (1983).

Yet, problems persist in establishing a stable, coalesced DC arc usingmultiple cathodes and/or plasma torches, that can be controlled to (1)permit injection of particulates into the plasma region for processingand permit ejection of these particles radially outwardly at a positionspaced from the anode and (2) control anodal heating. (As used herein,"plasma torch" shall mean a device consisting of a thin stick-typecathode surrounded by a tube shaped anode with a nozzle at one end. Whensuitable electric power is supplied to a plasma torch, a jet of plasmais emitted from the opening of the nozzle.) Problems with heatdissipation over the surface of the anode also continue to exist. It isdesirable to prevent localized extremely high temperature spots on theanode because hot spots cause detrimental evaporation of the anodematerial. Even when liquid cooled anodes are utilized, evaporation is aproblem.

The Harry articles referred to above describe use of a plurality ofcathodes spaced about a central axis for generating arcs in which plasmais formed, and U.S. Pat. No. 3,989,512, issued Nov. 2, 1976 to Saycealso describes a plurality of separate nontransferred arc plasma torchesarranged in spaced relation around a central axis for generating acolumn of plasma.

Background on plasma technology is also given in the article entitled"Plasma Technology And Its Application To Extractive Metallurgy" by S.M. L. Hamblyn, Mineral Science Engineering, SCI. Engng, Vol. 9, No. 3,pp. 151-176 (July 1977).

On Page 153 of the Hamblyn article, in FIGS. 6 and 7, there is a threephase (AC) plasma system stabilized by using a direct current supply.The arrangement forms a central plasma stream, and it is indicated thatthe device had a central gas flow along the plasma stream, to produce ahomogeneous volume of plasma. The system was used mainly forexperimental studies of heat transfer to refractory oxide in a fluidizedbed reactor. The device in this article does not provide for a coalescedDC arc, such as is obtained with the three cathodes utilized with thepresent device, nor does it teach the use of a gas stream for stablizingthe arcs, and/or material particle injection and processing.

Additional work is set forth in the article of C. Sheer et al entitled"Invited Review: Development And Application Of The High IntensityConvective Electrical Arc", Chem. Eng. Comm. Vol. 19, pp. 1-47 (1982),and in particular pages 17-27 include an explanation of a "cathode pump"that forms near a cathode tip. A single cathode is disclosed and in thearticle it is taught that particulate matter being introduced into theplasma is to some extent injected into the plasma, but for the most parttravels in the fringes of the plasma, and thus the high temperatureplasma core which is useful for processing is not fully utilized.

An additional patent which illustrates the general state of the art inplasma arc production of silicon nitride is Harvey et al. U.S. Pat. No.4,206,190, issued June 3, 1980. This describes a plasma arc furnace usedfor forming particulate silicon nitride.

Arc plasma dissociation of zircon is described in an article by thattitle in Chemical Engineering, Nov. 24, 1975. This just shows generalapplications of plasma technology.

Additional patents which illustrate use of plasma arcs and/or plasmagenerators are as follows:

    ______________________________________                                                                   Date                                               U.S. Pat. No. Inventor     of Issue                                           ______________________________________                                        3,313,908     R. Unger et al                                                                             04-11-67                                           3,496,280     D. Dukelow et al                                                                           02-17-70                                           3,573,090     J. Peterson  03-30-71                                           3,980,802     B. Paton et al                                                                             09-14-76                                           4,121,083     R. Smyth     10-17-78                                           4,141,694     S. Camacho   02-27-79                                           4,426,709     J. Fegerl et al                                                                            01-17-84                                           ______________________________________                                    

Although multiple cathode plasma generators are known in the prior art,several problems persist: (1) lack of stability of the plasma column anda tendency for the column to randomly dance across the anode surface;(2) localized hot spots on the anode surface that cause unwantedevaporation of the anode; (3) maintaining high heat flows to the anodewithout unwanted hot spots; (4) difficulty in overcoming the viscosityand thermal gradient effects of the plasma to inject particulate matterinto the plasma for processing; and (5) maintaining particulate matterwithin the plasma for sufficient times to ensure efficient and completeprocessing. It is to the solution of these problems that this inventionis directed.

SUMMARY OF THE INVENTION

A multiple source plasma generator arrangement, as shown by a multiplecathode arrangement, has the plasma creating members positioned about acentral axis. A gas nozzle is centered along the axis for feeding inertor reactive gases and/or particulate material from the top of the devicedownward toward the target surface. In the case of a multiple cathodearrangement, the location of the gas nozzle would be in the area inwhich the multiple cathodes are located and the nozzle would be directeddownward toward the anode. In a preferred form, the multiple arcarrangement is DC driven to provide a coalesced arc formed plasmacolumn, and a flow of inert gas from the nozzle is directed through thecolumn along its axis to enhance stability of the arc and to provide ameans for continuous feeding of particulate matter into the plasma corefor controlled processing of materials.

While it is well known that there is a self-induced pumping action (thecathode and anode jets described above) derived in the use of arcsgenerating plasma, the forced flow of gas in the present device indirection from the cathodes toward the anode strongly affects the plasmaflow. In the first embodiment of this invention, controlling the flow ofthe forced gas results in the creation of a bell shaped arc that has asubstantial area of contact with the anode to provide enhanced heattransfer to the anode without creating localized hot spots.

Additionally, the forced flow of gas from the central nozzle may be usedas a means for injecting particulate material into the core of themultiple arcs so that inflight processing, such as melting and/orchemical reactions of the materials, is confined within the plasma. Thisresults in higher processing efficiencies, and better control over theoperations.

A second embodiment comprises use of the invention as an arc plasmareactor through utilization of the interaction between forced gas flowsand self-induced gas flows that are created in this type of device. Theforced gas flows are externally applied convective gas flows in eitheror both the areas of the anode or cathodes. The self-induced flows arean electromagnetic pumping effect of gas from the cathode to the anode,called a cathode jet, and from the anode to the cathode, called an anodejet. Through utilization of the interaction of these self-induced andforced flows, plasma processing can be enhanced, for example, for makingfine powders from a variety of injected particulate material. The forcedflow from the cathodes toward the anode, augmented by the cathode jets,may be used to counteract the anode jet and form a stagnation layer at alevel between the cathodes and the anode. The stagnation regionrepresents a region of high plasma temperature and relatively low gasvelocities, which is ideally suited for plasma processing of particulatematerial. As will be shown, particulate material carried by the forcedgas flows is entrained in the high temperature plasma and maintained inthe stagnation region for a relatively long period of time to enhanceproper formation of the process products. The processed particulatematerial is then discharged from the arc region laterally outwardly andcan impinge against the sides of a furnace for collection. The forcedgas can be inert or a reactant for processing the particulate material.

In both embodiments of the invention, the interaction of the selfmagnetic fields of the arc leads to a coalescence of the individual arcsinto a single arc column. The coalescence occurs at some distance fromthe cathodes, so cathode-anode spacing is controlled to insure suchcoalescence. By proper regulation of the forced gas flow, the stagnationlayer will be at a location after the arcs have coalesced, and in aregion which provides enhanced material processing.

While a similar stagnation effect can be realized from a single cathodeusing a single arc, it does not have the advantage of enhanced particleentrainment due to the off-axis temperature peaks produced as a resultof the multiple cathode geometry. The multiple cathode arrangementprovides for enhanced processing of particles.

The combination of forced gas flows and the anode and cathode jets, andthe interaction of these flows, gives substantially better processingresults and efficiencies than those heretofore obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a plasma generator of thepresent invention including a schematic representation of the electricalcircuit used therewith;

FIG. 2 is a schematic side view of the device of the present inventionshowing the type of arc shape (bell shape) obtained with the forced gasflow used with the center nozzle for enhancing heat transfer to theanode;

FIG. 3 is a representation of the device of the present inventionshowing the effects of reduced forced gas flow that provides astagnation area in the plasma column between the anode and cathode forplasma reactor use;

FIG. 4 is a representation of the configuration of FIG. 3 illustratingparticulate processing when particles are injected through forced gasnozzles at both the anode and cathode ends of the plasma column;

FIG. 5 is a graph depicting typical results of operations of a device ofthe present invention illustrating the anode heat dissipation in thepresence of an anode jet dominated mode of operation as shown in FIG. 3,and illustrating the transition to a forced gas and cathode jetdominated mode of operation as shown in FIG. 2 at two arc lengths, whichillustrates the type of attachment of an arc to an anode at a standardcurrent of 100 amps and at different gas flows; and

FIG. 6 is a graph representing the voltage characteristics of a deviceof the present invention at two different arc lengths, and at varyinggas flows, plotting the voltage versus the gas flow with a standardcurrent of 100 amps.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, plasma forming means are indicated generally at 10,and in the form shown include an anode 11, and as shown in the preferredembodiment, a plurality of cathodes 12, 13 and 14 spaced from the anodea desired distance, and supported in a suitable manner (not shown).These can be supported in a frame that is suitably cooled, if necessary,and placed within the confines of a furnace, for example having wallsindicated schematically at 20.

In the form shown, a power supply 21 provides power to anode 11 througha line 22 and shunt current resistor 23, and then to anode 11. Each ofthe cathodes 12, 13 and 14 is connected to the negative terminal of thepower supply 21 through a line 26, shunt current resistor 24, andstabilizing resistor 25, in series with the resistor 24. Cathodes 12, 13and 14 are connected in parallel through line 26 to the negative side ofpower supply 21.

The above identified electrical arrangement provides adequatestabilization of current flow for establishing an arc. The shunt currentresistors provide a means of measuring current flow to monitor theoperation of the device.

In use, the arcs are generally initiated using a high frequency spark atvery close gaps, after which the cathodes can be moved away from theanode a desired amount to obtain the desired gap length.

The anode 11 preferably is copper or other suitable material. For use ina plasma reactor it can be water cooled to prevent it from beingconsumed or evaporated during operation. However, with a different anodeconfiguration than that shown in FIG. 1, it is possible to maintain anuncooled molten anode inside a crucible. It is also possible to producea coating on a substrate moving in the lateral direction which serves,at the same time, as an anode.

In the preferred form the three cathodes 12, 13 and 14 are positioned120° apart around a central axis 30 at an included angle along thelongitudinal axes of the cathodes of substantially 45°, as shown inFIG. 1. The central axis 30 is through the center of the anode 11, andthe tips of the cathodes 12, 13 and 14 are spaced at a desired distanceapart.

A gas nozzle indicated generally at 32 is centered along the centralaxis 30 and positioned in front of cathode 13. As shown in FIG. 2,nozzle 32 has a central opening 31 through which suitable inert orreactive gases can be directed down into the core of the plasma formed.

As shown in FIG. 2 in this form of the invention the cathodes 12, 13 and14 include suitable cathode holders or supports 34 and cathode tips 33.An arc is formed between the cathodes and the anode 35. For clarity,cathode 13 is not shown but is understood to be positioned behind nozzle32 as shown in FIG. 1. The plasma column 39 formed by the arc iscontrolled and stabilized by providing a flow of gas through the centralopening 31 of the nozzle 32 from a gas source 37 at a desired flow ratecontrolled by a valve 38. Valve 38 may be any suitable valve whichpermits varying the rate of gas flow. An inert gas such as argon may beused, and this will form a core 40 of material generally as outlined inthe dotted lines. The arcs from each of the cathodes 12, 13 and 14 willcoalesce approximately one-third of the way from the cathodes to theanode 35, the exact location depending on system parameters includingcathode/anode geometry, current flow, and forced gas flows, in thebell-shaped form shown to broaden out the area of contact of the arc onthe anode. The coalesced arcs form the plasma column 39. The jet of gascoming from the nozzle 32 tends to stablize the plasma column 39 andkeep it from dancing back and forth on the anode 35 as the arc isgenerated. The gas flow from the nozzle 32 will counteract the inducedgas flow from the anode 35 toward the cathodes 12, 13 and 14, commonlyknown as the anode jet, and will assist or add to gas flows from thecathodes 12, 13 and 14 to the anode 35, called the cathode jets.

In the form shown in FIG. 2, the bell shaped end 39A of the plasmacolumn 39 is obtained by having sufficient gas flow from the nozzle 32along the core 40 of the plasma column 39 to overcome self-induced anodejets moving in direction from the anode 35 toward the cathodes 12, 13and 14, and provide a net positive flow from the cathodes 12, 13 and 14to the anode 35. This gas flow then spreads out along the surface ofanode 35, forcing the plasma column to also spread. This results in thewide base of the plasma column which permits higher power arcs and hencegreater heat transfer without forming an extremely hot, localized pointcontact of the arc to the anode 35 which would cause excessivevaporization of the anode material. The gas flowing from nozzle 32 maybe inert for simple melting and/or vaporizing processes (e.g. coating,spherodization, etc.) or may be reactive for chemical transformations(e.g. reduction, coal gasification etc.)

If desired, particles (particulate matter) from a particle sourceindicated at 41 can be intermixed with the gas from the gas source 37.This particulate matter is thus injected into the plasma column 39 andis passed down through the column core 40, to intermix with the plasmaand chemically react, or be heated in a desired manner. The particlescan be carried in a gas stream from the source or dispersed in anysuitable manner. If desired, the particles can be processed in theplasma column 39 and then either deposited as a coating on the materialforming the anode 35 in the approximate area of contact between theplasma column and the anode or ejected as shown by arrows 42.

Thus, injecting a suitable column of gas along an axis positionedbetween a plurality of cathodes stabilizes the plasma column, andadjusting the flow of such gas diffuses the plasma column at its basewhere it contacts the anode to provide a bell shaped, wide area ofcontact between the plasma and the anode. In the form of the inventionshown in FIG. 2, the self-induced flow of gases from the anode to thecathodes that would occur in the case of a constricted plasma column atthe anode is overcome by the flow of the gas from the nozzle 32,augmented by the self-induced flow of gas from the cathodes to theanode. The flow from nozzle 32 can be adjusted with valve 38 between thegas source and the nozzle 32 to provide the desired bell shape.

As shown, the nozzle 32 is a water cooled sleeve having a partition tube46 surrounding the annular center tube 31, and the flow of water wouldbe cool along the walls forming the center tube 31. The water flows backout along the outer walls of the nozzle to keep the nozzle material fromdeteriorating in the high temperature environment of the furnace.

The same type of results can be obtained using a core gas flow forstabilization of plasma torch flows, if the cathodes are replaced withplasma torches. The anode 35 would remain an anode and the plasma jetsfrom the torches would be flowing toward the surface of the anode 35.Thus, the plasma torches would be operating in the transferred arc mode,where the anodes of the torches are electrically disconnected after arcinitiation. The use of a center inert gas core stabilizes the plasmaflow, and provides for a wider area of contact of the plasma to theanode as well as providing the ability to control processing ofparticles from a source by having better control of the time forprocessing and the temperatures.

In FIG. 3 a form of plasma column particularly useful for plasmaprocessing is shown which can be obtained by regulating the gas flowsthrough the valve 38. Again in FIG. 3, cathode 13 is not shown, but ispositioned behind nozzle 32. Self-induced gas entrainment from the anode47 occurs, as previously mentioned, without a flow of gas from thenozzle 32, so that there will be a convective effect upward from theanode 47 toward the cathodes 12, 13 and 14 forming an "anode jet"indicated at 52. The anode jet can dominate the plasma flow and evenresult in the plasma column extending upwardly above the ends of thecathodes 12, 13 and 14 in a direction away from the anode 47. However,with the nozzle 32 in operation and the gas flow adjusted to a desiredlevel, the plasma column indicated in this form of the invention at 50will not extend above the ends of the cathodes. The anode 47 will bewater cooled in a conventional manner for use in the apparatus of FIG.3, which forms a plasma reactor. In the operating mode shown in FIG. 3,gas flows from nozzle 32, augmented by the cathode jet flows shown at60, counteract the anode jet shown at 52. Where the flows meet, astagnation area is formed and the gas flows radially outward into thesurrounding atmosphere in the furnace. This results in an annular diskof plasma positioned between the ends of the cathodes 12, 13 and 14 andthe anode 47, for example in the disk-like stagnation zone indicated at51. Essentially what occurs is that the forced gas flow from the nozzle32 combined with the flow of the cathode jets 60 oppose the anode jetgas flow indicated by the arrows 52. Where the pressures equalizestagnation zone 51 results, causing an outflow of gases indicated by thearrows 53. This outflow is directed radially outward from around thecircumference of the plasma column 50. The stagnation zone 51 may bemoved axially in relation to the cathodes 12, 13 and 14 and anode 47 asdesired by adjusting the parameters of operation including cathode/anodegeometry, current flow, and forced gas flow. As used, the primary meansof controlling the location of the stagnation zone is through adjustingvalve 38 to change the forced gas flow through nozzle 32. While in thismode of operation there is a small area of arc contact on the anode thatcauses localized heating, the anode is additionally cooled by theself-induced anode flow 52 along the anode surface.

In the case where it is desired to use the device as an arc plasmareactor for chemical processing of fine powders, the arrangement shownin FIG. 4 can be used to process particulate matter 54. Particulatematter from a particle source 41 is introduced into the gas flow in thenozzle 32 and is thereby carried down into the plasma column 50. At thesame time small amounts of gas from self-induced flow at the anode willbe moving upwardly as previously explained. Additional particles can beinjected into the anode jet gas flow 52 from the anode to cathode withnozzles indicated at 56 lying parallel to the surface of anode 47. Theseadditional particles then will move up into the plasma column 50 alongwith the self-induced anode jet flow so that the center core of theplasma column will be filled with particles 54 as illustrated in FIG. 4.The particles 54 then will be capable of being processed by the plasmafor a period of time that is longer than that which is possible withoutthe stagnation zone 51 generated by the forced gas flow from nozzle 32in combination with the cathode jets 60 and anode jet 52. The process isused to cause the particles to change in character, for example, formingspheres from irregularly shaped particles. Additional chemical reactionscan take place or the particles can be broken into fine powdersdepending on the reaction with the plasma column. Further, the sidewalls 20 of the furnace will be bombarded with the processed particles54A and may be used to collect the materials being processed, utilizingboth the forced gas flow from the nozzle 32 and the natural pumpingaction of the plasma column 50.

The particulate material is injected into the plasma column 50 by thepumping action of either 15 or both the cathode jets 60 and anode jet52, aided by the forced gas flows in nozzles 32 and 56. These pumpingactions and forced gas flows overcome the effect of the high temperaturegradients, as well as the high viscosity of the plasma, which otherwiseinhibit the injection of particles. The geometry of the multiple cathodearrangement as shown in FIG. 4 also enhances the injection of particlesby feeding particles into a region where they are constrained by thenoncoalesced plasmas resulting from the individual cathodes and henceare injected into the coalesced plasma core. The pumping action alsopermits feeding particles in both directions from the cathode and theanode, and the processed product is ejected radially at the stagnationregion.

When desired, the heat dissipated at the anode can be made to increasesignificantly where there is a diffused bell attachment of the arc orplasma column, such as that shown in FIG. 2. Also, by controlling theforced gas flow from nozzle 32 the stagnation region or disk can beformed at a chosen location above the anode.

In FIG. 5, heat transfer to the anode in presence of an anode jet isplotted for two different arc lengths. One is a relatively short 5millimeter arc length, and the other is a 40 millimeter arc length.Using the triple cathode arrangement with a current of 100 amps througheach cathode, it can be seen that at a forced gas flow from nozzle 32 inthe

range of 14 grams per minute, and an arc length of 40 millimeters, thereis a pronounced shift in the heat dissipation. The approximate locationof the change from spot attachment of the arc (anode jet dominated) tobell shaped attachment (forced gas and cathode jet dominated) isindicated on FIG. 5.

At shorter arc lengths, such as 5 millimeters, a forced gas flow fromnozzle 32 in the range of 10 to 12 grams per minute resulted in a changefrom the spot attachment of the plasma column to the anode, as shown inFIG. 3, to a diffused bell-shaped attachment, as shown in FIG. 2. Again,a reasonable arc length is desired, and the voltage levels at 100 ampscurrent ranged from approximately 13 volts for the five millimeter arclength, to about 30 volts for the longer arc length of 40 millimeters.At arc lengths between these extremes the voltage is also between thesevalues, and varies somewhat. Generally speaking, the shift from the spotattachment to the diffused bell type of attachment caused a shift inheat dissipation that was discernable and occurred at forced gas flowsbetween 10 and 14 grams per minute from the nozzle 32. This is with thecurrent and voltage shown, and the triple cathode arrangement.

As shown in FIG. 5, at shorter arc length the stagnation region isformed at lower gas flows. The gas flows can be adjusted to obtain thedesired results for a given arc length and a given arc current. Currentalso can be varied as desired to obtain the desired attachment point.

The labeling for the respective curves in FIG. 5 shows that the heatflow to the anode commences to go up as the anode attachment areaincreases, and that the diffused bell attachment causes higher heatflows to the anode.

FIG. 6 is a graph generally illustrating the effect of various gas flowsfrom the nozzle 32 on the voltage levels to maintain a current of 100amps to each of the cathodes. This shows the various voltage levels thatare produced by the conditions illustrated in FIG. 5, in presence of ananode jet, at particular currents for two different arc lengths andshows the approximate region, as marked on FIG. 5, of formation of thediffuse bell attachment to the anode at different gas flows.

Experiments have shown that with a triple cathode the voltage will varyas current is changed. When the current increases into the range of 80amps, the arcs will coalesce and form one large arc column, asmentioned. The cold gas from the nozzle 32 will penetrate the plasma inthe center, and will tend to cool the plasma in the core. This, however,has little effect on the arc conductance, and the arc voltages remainfairly stable from about 80 amps and higher in experimentation. Further,each of the cathodes draws substantially the same current, particularlywhere there is a coalesced arc.

With the invention it has been found that the greater the arc length,the higher the voltage. With a cathode jet and forced gas dominated modeof operation, that is, with gas flows in nozzle 32 sufficient toovercome the anode jet, the optimum operating point, where the threearcs merge together and where the energy dissipation is also the lowest,generally is in the range of 80 to 100 amps and at a low voltage point.In this mode of operation the diffuse bell attachment occurs. Theinjected particles, when such particles are injected, will be bestentrained into the hot plasma core at the point where the arcs coalesce.

The use of the three cathodes adds stability to the coalesced arc, andprovides for a large plasma volume through which the particles may passfor treatment.

The arc voltage generally increases with increasing forced gas flow ratefrom nozzle 32 at each arc length. As the gas flow from the nozzle 32increases, the cooling effect on the plasma becomes stronger. Thiscooling of the plasma increases the resistance of the arc and the arcvoltage must rise to compensate. The voltage characteristics, generallyspeaking, of the triple cathode arc are similar to those found inconventional single cathode arcs, and because the forced gas from thenozzle 32 acts as an artificial cold cathode jet, the forced gas aids inthe stabilization of the arc.

In the anode jet dominated mode of operation, before the diffuse bellattachment occurs, the flow from the anode to cathode causes the disk ofplasma at the stagnation region 51 spaced from the anode. In FIG. 5, theanode jet dominated mode is indicated at low gas flows for the differentarc lengths shown there, and the diffused bell attachment of the anodeoccurs where the greatest amount of heat is shown to be dissipated for agiven amount of power. It should be noted that for some limited valuesof current, voltage, and distance of cathodes, the stagnation zone 51shown in FIGS. 3 and 4 occurs as a result of the cathode jet alonewithout forced gas flow through nozzle 32; however, the flow throughnozzle 32 allows creation of the stagnation zone for a much greaterrange of current, voltage, and distance of cathodes and in any eventprovides for greater control over the creation and location of thestagnation zone. The forced gas flow through nozzle 32 under typicaloperating conditions is approximately five times higher than theself-induced flow of the cathode jets.

The presence of a self-induced cathode jet 60 depends in part upon theshape of the cathode tip. If the cathode tip is blunt, rather than afine or sharp tip, the cathode jet is reduced and it is possible tooperate in an anode jet dominated mode where the anode jet is in thefull reverse flow. An anode jet flow, in the opposite direction of theelectron flow, aids the positive ion flow. When the anode jet is forceddownward toward the anode by the combination of the forced gas flow andcathode jets, the flow favors the electron flow, resulting in lower arcvoltages. When an anode jet, and a flow in the reverse direction areimpinging upon each other, a high potential drop is needed to push theparticles through the stagnation layer.

The anode jet hinders heat transfer to the anode as a result of the flowalong the anode and up through the plasma column. Thus, from the viewpoint of heat transfer to the anode, the anode jet mode should beavoided, and as shown in FIG. 5, for example, a sufficient flow of gasfrom the cathode should be provided through the central core to providethe diffuse bell attachment to the anode. However, when the flows are inthe anode jet mode, and the forced flow through the nozzle 32 isselected to form a stagnation zone, the ability to inject particles atboth the anode and through the central nozzle is present, and theprocessing of the particles in the stagnation zone at the plasmaprovides for greater length of time of particle processing, higherheats, and in many instances greater efficiency. The introduction of theforced gas down the central axis between the cathodes at a controlledrate provides a very stable arc under all conditions, and at selectedforced gas flows will increase the anode heat transfer, and at other(lower) forced gas flows can create a stagnation zone between the anodeand cathode which results in enhanced conditions for processing ofparticulate matter. Thus, the flows from nozzle 32 for obtaining theoperating mode shown in FIGS. 3 and 4 will be below the flow that causesbell attachment shown in FIG. 2.

Varying the current provides an additional means of control over theformation and location of the stagnation zone. Lower currents reduce theintensity of the cathode jets and hence will allow the stagnation zoneto move somewhat closer to the cathodes; however, as noted above, theeffect of the cathode jets are significantly less than the effect of theforced flow of gas.

The position of the stagnation region can be adjusted by regulating thepressure through the cathode forced gas nozzle 32, as well as the anodeforced gas nozzles 56.

The carrier gas used for injection of particulate matter into the arcmay be inert or it may represent a chemical reactant depending on thedesired reaction in the plasma.

This device may be used for physical as well as for chemical processing.Physical processing may, for example, include spheroidization ofirregularly shaped powder particles, production of ultrafine powders(for example silica) and densification of conglomerates. Chemicalprocessing includes the decomposition of compounds (for example toxicwastes), the synthesis of carbides, nitrides and refractory metaloxides, the production of chemical compounds and alloys using solids,liquids or gases as starting materials.

Radial ejection of the products from the hot plasma zone provides for afast quench of the products which may be further enhanced by impingmentof the products on water cooled walls or by entrainment of the productsinto water cooled channels.

The injection nozzles 56 project radially from the axis of the plasmacolumn, and perhaps six or so equally spaced nozzles would be usedaround the axis. The particles from nozzles 56 will generally beconveyed in a gaseous fluid in a known manner.

While the device will work with only one cathode, the use of threecathodes provides for better control and better action. The device willalso work with modifications including the height of the gas nozzlerelative to the cathode tips, the rotation of the entire multiple arcplasma torch assembly in any direction, and different anode geometriessuch as stick anodes, ring anodes, or anode crucibles. In a preferredembodiment of this invention, the particle injection occurs at a pointequal to or below the cathode tips to prevent cathode degradation.

Although the present invention has been described with reference topreferred embodiments, workers skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention.

What is claimed is:
 1. An apparatus for plasma processing comprisingmeans forming a plasma column directed toward a surface comprising aplurality of substantially equally spaced sources of plasma arrangedannularly around a central axis, which sources coalesce to form theplasma column, a source of a gas under pressure, a nozzle for directinggas flow from the source, the nozzle having a flow axis substantiallycoinciding with said central axis and the flow being directed in thesame direction as the flow of plasma from the sources of plasma, saidnozzle being in a region adjacent the sources of plasma and where theplasma has not coalesced, and means for regulating flow of gas from thesource through said nozzle to provide a gas flow sufficient to stabilizeand control the configuration of the plasma column formed.
 2. Theapparatus of claim 1 wherein said means forming a plasma streamcomprises at least three cathodes substantially equiangularly displacedabout the Central axis forming an arc with respect to an anode, saidanode comprising said surface toward which the plasma column isdirected.
 3. The apparatus of claim 1 wherein air means forming a plasmacolumn comprises three sources of plasma arranged about the central axisat substantially 120° relative to each other.
 4. The apparatus asspecified in claim 1 wherein the surface is a heat dissipation surface,and said means for regulating flow of gas from the source is adjusted tocause the gas flow in direction form the sources of plasma toward thesurface to be the dominant flow on the interior of the plasma column,and to cause the column to spread out over an area of said surfacesubstantially larger than the area of the plasma column for the majorityof its length above the surface.
 5. The apparatus as specified in claim3 wherein said means forming a plasma column comprise an anode havingsaid surface, and three cathodes spaced form said anode a desired amountto provide an arc and to create a self-induced flow of gas from theanode toward the cathode, said means for regulating flow of gasproviding a flow of gas through the nozzle tending to overcome theself-induced gas flow to form a stagnation zone in the column spacedfrom said surface of said anode, and also spaced from said cathodes indirection toward aids anode.
 6. The apparatus as specified in claim 5wherein said cathodes are spaced from said anode a distance so that thearcs formed coalesce to form a single plasma column, and said nozzledirects a flow of gas through the center of the coalesced arcs, the gasflow through said nozzle being adjusted to provide a disk of plasmaspaced from the anode, and also spaced, from the cathodes and radiatingfrom the coalesced arc, and surrounding the rest of the plasma column,and means for providing particles to said gas flow whereby saidparticles are discharged outwardly around said disk with the gas fromsaid nozzle.
 7. The apparatus as specified in claim 2 wherein thecathode and anode are positioned and said flow of gas is adjusted topermit the arc to operate in an anode jet dominated mode, with flow ofgas from the anode toward the cathode, and the flow of gas from saidnozzle opposes the anode jet to form a discharge disk of plasma throughthe column of plasma between the cathode and the anode, and means toinject particles into the gas carried through the nozzle, and also intothe anode jet flow.
 8. An apparatus for processing particles utilizing aplasma column directed in a first direction comprising:a plurality ofcathodes mounted to be spaced laterally from a central axis and beingspaced annularly from each other and at a substantially equal distancefrom the central axis; an anode spaced from the cathodes in the firstdirection along the axis, said cathodes each being substantially equaldistances from the anode; means energizing the cathodes tosimultaneously form individual arcs from each cathode, said cathodesbeing spaced from the central axis so that the individual arcs coalescesto form a plasma column extending to the anode; a source of gas underpressure; a nozzle connected to receive a flow of gas from the sourceand being positioned relative to the cathodes such that the nozzle is ina region where the plasma arcs forming the plasma column have notcoalesced, the nozzle having a flow axis substantially coincident withthe central axis and providing a gas flow toward the anode; means forregulating the flow of gas from said nozzle to form a stable gas core inthe plasma column that is formed after the arcs have coalesced, and tocontrol the stability of the plasma column primarily by the gas flowfrom the means for regulating; and a source of particles to be processedconnected to the nozzle to add particles into the regulated gas flowcoming form the nozzle to move with the regulated gas flow in the firstdirection from the nozzle axially in the center portions of the plasmacolumn, said particles being carried into the central core of the plasmacolumn with the regulated gas flow from the nozzle.
 9. The apparatus asspecified in claim 8 wherein said means for regulating the gas flowcontrols the plasma column to have a wider portion at a location betweenthe cathodes and the anode, at least portions of said particles therebybeing discharged radially out from the plasma column with gas flow atthe location of the wider portion.
 10. The apparatus as specified inclaim 9 and a second source of particles provided at said anode, saidanode defining a surface along which an induced gas flow is formed intothe plasma column, and said second source of particles providingparticles in said induced gas flow.